Ian Grant said:
Yes different but derived from the two.
To quote "The Zeiss Protar (1890) which combined in effect the front half of a Rapid Rectilnear (old Achromat) with the rear half of the Concentric (new Achromat), and yielded a wider field, was the first lens to bear the name "Anastigmat".
So not really so different after all. Then the Double Protar is a symmetrical lens each half of the lens again derived from a reworked combination of RR & Concentric elements again and each half is a full lens in it's own right, and of course thats why they became so versatile.
If you look at the lens designs it's very clear.
I should add that our lenses are almost certainly Double Protars and very different to the original Protar. Hence why so diferrent to your Concentric. Should add that the Protars, Dagors, Double Protars etc were a major advance on RR & Concentric lenses optically.
Protar type lenses are still manufactured, unfortunately no Dagors . . . . .
Ian,
You have a point there, but my point remains:
By saying "in effect", it was referring to the actual doublet construction. At first the Chevalier-type doublet had a positive element on the convex side and a negative on the concave or flat side, then the Grubb-type doublet reversed this arrangement with a negative element on the convex side which became more or less the norm. The Aplanat-type lens combined two relatively thin Grubb-type doublets arranged around a central stop, but it came out decades before the introduction of the Jena glass. In fact the first application of Jena glass to photographic lenses was by Voigtlander who employed it to a version of the Euryscope, its Aplanat-type lens, the benefit was most doubtful as it could not eliminate astigmatism at all.
The Ross Concentric, while often claimed as the first anastigmat design employing Jena glass but failed to hit the market until 1892, did not prove satisfactory because it had astigmatism and field curvature corrected, but neglected to correct for spherical aberration: sort of two steps forward but two steps back. The Concentric was also purely symmetrical with a pair of closely-spaced doublets of Chevalier-type.
I do not know the source from which your quote was taken, but I presume it was not from contemporary official sources; all I have in my possession were copies of original catalogues (Zeiss 1891 and 1902), contemporary dealers catalogues, contemporary reviews, actual examples and test negatives I did myself.
But there again it is often assumed that the asymmetrical Anastigmat/Protar was invariably a four-element lens; some versions actually had a cemented triplet at the back. While the front doublet remained a Grubb-type using the then conventional crown and flint combination, it has very little positive power, the rear group used one (or two, in the case of versions with a triplet rear) Jena baryta crown, partnered with a flint negative, to produce an anastigmatically flat field and most of its positive power.
The Double-Protar is again a completely different kettle of fish altogether; which can be understood as having the whole Protar squashed into one lump and put to one side of the iris, with another similar (or identical) unit fitted to the other side.
